Polymeric materials that reversibly change shape, size, and/or mechanical properties in response to external stimuli have attracted considerable interest due to their potential applications as actuators for biomedical and mechanical purposes. Responsive polymeric materials can be divided into three classes: (1) electro-active polymers (Jager et al., Science 2000, 290:1540; Osada et al., Nature 1992, 355:242); (2) light or thermal responsive elastomers (Lendlein et al., Science 2002, 296:1673; Camacho-Lopez et al., Nature Mater. 2004, 3:307; Yu et al., Nature 2003, 425:145; Kim, et al., Science 2012, 335:1201); and (3) pH or solvent responsive gels (Sidorenko et al., Science 2007, 315:487; Beebe et al., Nature 2000, 404:588; Chen, et al., Nature 1995, 373:49; Caldorera-Moore et al., Acc. Chem. Res. 2011, 44:1061).
Many organisms use water-sorption-induced swelling for actuation (Fratzl et al., Nature 2009, 462:442). Several types of water-responsive hydrogels have been developed for actuator fabrication (Sidorenko et al., Science 2007, 315:487), but they exhibit slower response, lower stress generation and marginal stability in comparison to animal muscle fibers.
Polypyrrole (PPy) is an electro-active polymer with many desirable properties that could allow it to act as an artificial muscle (Smela, Adv. Mater. 2003, 15:481). PPy can also absorb water and change its shape, which is the basis for driving motion in a rotary actuator (Okuzaki et al., J. Polym. Sci. Polym. Phys., 1998, 36:2237). However, the PPy rotary actuator outputs little mechanical force or power, in contrast to PPy-based electro-actuators. Okuzaki's actuator, based on a small anion-doped polypyrrole (PPy/ClO4), responds due to the polypyrrole's water-sensitivity, which is mainly a physical sorption/desorption process and has a weak impact on the polymer's mechanical properties. The weak physical sorption/desorption process in Okuzaki's actuator leads to weak force generation and low water-induced stress. The maximum water-induced bending force in Okuzaki's actuator is only 4.5 times its own weight. The maximum water-induced strain of Okuzaki's actuator is 0.36%. Water sorption/desorption induced bending is slow. Therefore, the low stress generation of Okuzaki's actuator limits its potential for commercial applications.
There is a need for a water-based actuator capable of generating suitable levels of mechanical force or power for commercial applications.
It is therefore an object of the invention to provide materials capable of generating suitable levels of mechanical force in response to moisture and methods of making and using thereof.
It is further an object of the invention to provide materials capable of rapid moisture-induced actuation and methods of making and using thereof.
It is an additional object of the invention to provide devices using these materials.
It is also an object of the invention to provide methods of using these material and devices for performing work and/or power generation.